An electron avalanche is a process in which a number of free electrons in a medium (usually a gas) are subjected to strong acceleration by an electric field, ionizing the mediums' atoms by collision (called impact ionization), thereby forming "new" electrons to undergo the same process in successive cycles. Electron avalanches are essential to the dielectric breakdown process within gases. The process can culminate in corona discharges, streamers, leaders, or in a spark or continuous arc that completely bridges the gap. The process extends to huge sparks — streamers in lightning discharges propagate by formation of electron avalanches created in the high potential gradient ahead of the streamers' advancing tips. Once begun, avalanches are often intensified by the creation of photoelectrons as a result of ultraviolet radiation emitted by the excited medium's atoms in the aft-tip region. https://www.chemeurope.com/en/encyclopedia/Electron_avalanche.html
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